Cyclic nucleotides, especially cyclic GMP, are important in the visual process and in other functions of the retina and pigment epithelium. Deficiencies in the enzymes of cyclic nucleotide metabolism may also be responsible for diseases related to retinal dysfunction. In a dog model for retinitis pigmentosa it appears that a switch in cyclic GMP phosphodiesterase (PDE) fails to occur during development in retinas of affected animals. This coupled with low levels of calmodulin in affected retinas, results in abnormally high cyclic GMP concentration and photoreceptor degeneration. Protein kinase activity is also important in the photoreceptor and can be controlled by cation and nucleotide concentration as well as several other modulators naturally present in outer segments.